In recent years, we have witnessed the proliferation of wireless voice telecommunication networks. Unfortunately, due to RF channel impairments and use of speech compression techniques, voice quality in the wireless network is not as good as it is in the wireline network. There is therefore a thrust to provide better voice quality in wireless communications.
In wireless networks, channel bandwidth is at a premium. There is therefore an advantage in compressing voice signals in digital format by an apparatus called a vocoder. The name "vocoder" stems from the fact that its applications are specific to the encoding and decoding of voice signals primarily. Vocoders are usually integrated in mobile telephones and the base stations (or a link therefrom) of the telecommunication network. They provide compression of a digitized voice signal as well as the reverse transformation. Pulse Code Modulation (PCM) is an example of a signal digitization technique. The main advantage of compressing speech is that it uses less of the limited channel bandwidth for transmission. The main disadvantage is some loss of speech quality.
In wireless telecommunications, the determination of quality of service (QOS) objectives is an issue that significantly affects voice quality. In the case of digital transmission, quality of service is usually measured by calculating the frame error rate (FER). FER is simply a measure of the number of frames in error during a certain period of time. QOS is therefore controlled by altering certain parameters of the network, which influence the Frame Error Rate (FER). It can be improved by raising the energy-per-bit to noise ratio (Eb/No). In the case of analog transmission, quality of service is usually measured by calculating the signal-to-noise-ratio (SNR). The principal parameter for controlling QOS is the variable power settings of the signal transmitters in the wireless terminals and the base stations. Through established methods of signaling and control, these power settings are adjusted depending on many factors, namely: the distance between the wireless terminal and the base station, interference on the RF channel, etc.
Another parameter that may be changed to influence QOS is the use of different Forward error correction (FEC) techniques for data packets transported over the RF channels. That is, some FEC techniques are better than others are (and to some extent more complicated and more costly to implement) and using them helps to improve QOS. Several FEC techniques are briefly described later in this specification.
Yet another parameter that may be changed to influence QOS is frequency reuse (also called frequency planning). Wireless telecommunication networks rely on an intelligent allocation and reuse of channels throughout a coverage region. Each base station is allocated a group of RF channels to be used within a small geographic area called a cell. Base stations in adjacent cells are assigned channels that are different from those of its neighbors. The base station antennas are designed to achieve the desired coverage within the specified cell. By limiting the coverage area to the boundaries of a cell, the same group of channels may be used to cover different cells separated from each other by distances large enough to keep interference levels at a minimum. The design process of selecting and allocating channel groups for all the base stations within a network is frequency reuse or planning.
In actual telecommunications systems, QOS is usually set at a specified level for the end-to-end connection. Typically, this implies that there is only one radio link (e.g. a wireless-to-wireline call). The system is therefore designed for a connection involving only one RF link. However, for a connection involving more than one RF link (e.g. a wireless-to-wireless call), leaving the QOS objectives at the same level as for a connection involving only one RF link results in a particular problem. In effect, with the same QOS objectives, a wireless-to-wireless connection, involving two radio links, will result in approximately twice as many errors permitted for the data frames as would a wireless-to-wireline connection, involving only one radio link. This situation can be remedied simply by determining the nature of the end-to-end connection and then setting the quality of service objectives for each radio link accordingly. This will result in enhanced speech quality in wireless-to-wireless links.
Thus, there is a need in the industry to provide a method and an apparatus for improving the voice quality of wireless connections by controlling the quality of service objectives depending on the nature of the end-to-end telecommunication link.